This study aims to describe components and levels of upper secondary social science students' reasoning about social problems. We consulted conceptualizations of social scientific reasoning in sociology textbooks and social science education literature, analyzed student papers, and conducted focus groups with social science teachers and teacher educators to define social scientific reasoning by proficiency levels and identify common flaws in students' reasoning. The papers were written by upper secondary social science students from eight schools in the Netherlands. We defined social scientific reasoning in terms of three components (describing, explaining, and solving problems) and five reasoning activities (causal analysis; use of social scientific concepts, models, and theories; use of evidence; use of perspectives and reflections on them; and comparing). We described these reasoning activities in three proficiency levels supported by practical examples and rubrics for students' reasoning. These insights can inform teachers and teacher educators in monitoring students' progression and designing teaching materials and activities that can promote students' social scientific reasoning.